block/nvme: nvme_process_completion() fix bound for cid

NVMeQueuePair::reqs has length NVME_NUM_REQS, which less than
NVME_QUEUE_SIZE by 1.

Fixes: 1086e95d ("block/nvme: switch to a NVMeRequest freelist")
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Maksim Davydov <davydov-max@yandex-team.ru>
Message-id: 20231017125941.810461-5-vsementsov@yandex-team.ru
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>